Pycnometry is the determination of density through measuring the volume of solids accurately.
A gas pycnometer is a laboratory device used for measuring the density — or more accurately the volume — of solids, whether they are regularly shaped, porous or non-porous, monolithic, powdered, granular or in some way comminuted. It employs Boyle’s Law, the relationship of gas displacement and the volume:pressure relationship. A gas pycnometer is also sometimes referred to as a helium pycnometer.
While pycnometers are recognized as devices for measuring solid density they actually measure volume. Density is calculated as the ratio of mass to volume; with mass being measured on a discrete device, i.e. by weighing. The pycnometer uses a chamber of known volume in to which the sample is placed. The chamber is then filled to capacity with Helium, the volume of the sample being the reduction in volume of gas required to fill the chamber. The volume measured depends on the atomic or molecular size of the gas used, the finer the gas the more the surface roughness will be covered. Helium is therefore ideal as the measurement gas, as it is of small size and also inert.
Closed pores, i.e. those that do not communicate with the surface of the solid, are included in the measured volume. Helium may however demonstrate some measurable permeability through low density solids (polymers and cellulosic materials predominantly) thus interfering with the measurement of solid volume. In such cases larger molecule gases such as nitrogen or sulphur hexafluoride are beneficial.
Adsorption of the measuring gas should be avoided, as should excessive vapour pressure from moisture or other liquids present in the otherwise solid sample.
Gas pycnometers are used extensively for characterizing a wide variety of solids such as heterogeneous catalysts, carbons, metal powders, soils, ceramics, active pharmaceutical ingredients (API’s) and excipients, petroleum coke, cement and other construction materials, cenospheres/glass microballoons and solid foams.